mdadm: A New Tool For Linux Software RAID Management

raidtools has been the standard software RAID management package for Linux since the inception of the software RAID driver. Over the years, raidtools have proven cumbersome to use, mostly because they rely on a configuration file (/etc/raidtab) that is difficult to maintain, and partly because its features are limited. In August 2001, Neil Brown, a software engineer at the University of New South Wales and a kernel developer, released an alternative. His mdadm (multiple devices admin) package provides a simple, yet robust way to manage software arrays. mdadm is now at version 1.0.1 and has proved quite stable over its first year of development. There has been much positive response on the Linux-raid mailing list and mdadm is likely to become widespread in the future. This article assumes that you have at least some familiarity with software RAID on Linux and that you have had some exposure to the raidtools package.

mdadm has five major modes of operation. The first two modes, Create and Assemble, are used to configure
and activate arrays. Manage mode is used to manipulate devices in an active array. Follow or Monitor
mode allows administrators to configure event notification and actions for arrays. Build mode is used when
working with legacy arrays that use an old version of the md driver. I will not cover build mode in this
article. The remaining options are used for various housekeeping tasks and are not attached to a specific
mode of operation, although the mdadm documentation calls these options Misc mode.

Creating an Array

Create (mdadm --create) mode is used to create a new array. In this example I use mdadm to create a RAID-0 at /dev/md0 made up of /dev/sdb1 and /dev/sdc1:

The --level option specifies which type of RAID to create in the same way that raidtools uses the raid-level configuration line. Valid choices are 0,1,4 and 5 for RAID-0, RAID-1, RAID-4, RAID-5 respectively. Linear (--level=linear) is also a valid choice for linear mode. The --raid-devices option works the same as
the nr-raid-disks option when using /etc/raidtab and raidtools.

In general, mdadm commands take the format:

mdadm [mode] <raiddevice> [options] <component disks>

Each of mdadm's options also has a short form that is less descriptive but shorter to type. For example, the
following command uses the short form of each option but is identical to the example I showed above.

# mdadm -Cv /dev/md0 -l0 -n2 -c128 /dev/sdb1 /dev/sdc1

-C selects Create mode, and I have also included the -v option here to turn on verbose output. -l and -n
specify the RAID level and number of member disks. Users of raidtools and /etc/raidtab can see how much easier it is to create arrays using mdadm. You can change the default chunk size (64KB) using the --chunk
or -c option. In this previous example I changed the chunk size to 128KB. mdadm also supports shell
expansions, so you don't have to type in the device name for every component disk if you are creating a
large array. In this example, I'll create a RAID-5 with five member disks and a chunk size of 128KB:

This example creates an array at /dev/md0 using SCSI disk partitions /dev/sda1, /dev/sdb1, /dev/sdc1, /dev/sdd1, and /dev/sde1. Notice that I have also set the chunk size to 128 KB using the -c128 option.
When creating a RAID-5, mdadm will automatically choose the left-symmetric parity algorithm, which is the best choice.

Use the --stop or -S command to stop running array:

# mdadm -S /dev/md0

/etc/mdadm.conf

/etc/mdadm.conf is mdadms' primary configuration file. Unlike /etc/raidtab, mdadm does not rely on
/etc/mdadm.conf to create or manage arrays. Rather, mdadm.conf is simply an extra way of keeping track
of software RAIDs. Using a configuration file with mdadm is useful, but not required. Having one means
you can quickly manage arrays without spending extra time figuring out what array properties are and
where disks belong. For example, if an array wasn't running and there was no mdadm.conf file describing it,
then the system administrator would need to spend time examining individual disks to determine array
properties and member disks.

Unlike the configuration file for raidtools, mdadm.conf is concise and simply lists disks and arrays. The
configuration file can contain two types of lines each starting with either the DEVICE or ARRAY
keyword. Whitespace separates the keyword from the configuration information. DEVICE lines specify a
list of devices that are potential member disks. ARRAY lines specify device entries for arrays as well as
identifier information. This information can include lists of one or more UUIDs, md device minor numbers,
or a listing of member devices.

In general, it's best to create an /etc/mdadm.conf file after you have created an array and update the file when new arrays are created. Without an /etc/mdadm.conf file you'd need to specify more detailed information about an array on the command in order to activate it. That means you'd have to remember which devices belonged to which arrays, and that could easily become a hassle on systems with a lot of disks. mdadm even provides an easy way to generate ARRAY lines. The output is a single long line, but I have broken it here to fit the page:

If there were multiple arrays running on the system, then mdadm would generate an array line for each one.
So after you're done building arrays you could redirect the output of mdadm --detail --scan to
/etc/mdadm.conf. Just make sure that you manually create a DEVICE entry as well. Using the example I've
provided above we might have an /etc/mdadm.conf that looks like:

Starting an Array

Assemble mode is used to start an array that already exists. If you created an /etc/mdadm.conf you can
automatically start an array listed there with the following command:

# mdadm -As /dev/md0
mdadm: /dev/md0 has been started with 2 drives.

The -A option denotes assemble mode. You can also use --assemble. The -s or --scan option tells mdadm to look in /etc/mdadm.conf for information about arrays and devices. If you want to start every array listed in /etc/mdadm.conf, don't specify an md device on the command line.

If you didn't create an /etc/mdadm.conf file, you will need to specify additional information on the command line in order to start an array. For example, this command attempts to start /dev/md0 using the devices listed on the command line:

# mdadm -A /dev/md0 /dev/sdb1 /dev/sdc1

Since using mdadm -A in this way assumes you have some prior knowledge about how arrays are arranged,
it might not be useful on systems that have arrays that were created by someone else. So you may wish to
examine some devices to gain a better picture about how arrays should be assembled. The examine options
(-E or --examine) allows you to print the md superblock (if present) from a block device that could be an
array component.

mdadm's examine option displays quite a bit of useful information about component disks. In this case we
can tell that /dev/sdc1 belongs to a RAID-0 made up of a total of four member disks. What I want to
specifically point out is the line of output that contains the UUID. A UUID is a 128-bit number that is
guaranteed to be reasonably unique on both the local system and across other systems. It is a randomly
generated using system hardware and timestamps as part of its seed. UUIDs are commonly used by many
programs to uniquely tag devices. See the uuidgen and libuuid manual pages for more information.

When an array is created, the md driver generates a UUID for the array and stores it in the md superblock.
You can use the UUID as criteria for array assembly. In the next example I am going to activate the array
to which /dev/sdc1 belongs using its UUID.

This command scans every SCSI disk (/dev/sd*) to see if it's a member of the array with the UUID 84788b68:1bb79088:9a73ebcc:2ab430da and then starts the array, assuming it found each component device. mdadm will produce a lot of output each time it tries to scan a device that does not exist. You can safely ignore such warnings.

Managing Arrays

Using Manage mode you can add and remove disks to a running array. This is useful for removing failed
disks, adding spare disks, or adding replacement disks. Manage mode can also be used to mark a member
disk as failed. Manage mode replicates the functions of raidtools programs such as raidsetfaulty, raidhotremove, and raidhotadd.

For example, to add a disk to an active array, replicating the raidhotadd command:

# mdadm /dev/md0 --add /dev/sdc1

Or, to remove /dev/sdc1 from /dev/md0 try:

# mdadm /dev/md0 --f ail /dev/sdc1 --remove /dev/sdc1

Notice that I first mark /dev/sdc1 as failed and then remove it. This is the same as using the raidsetfaulty and raidhotremove commands with raidtools. It's fine to combine add, fail, and remove options on a single command line as long as they make sense in terms of array management. So you have to fail a disk before removing it, for example.

Monitoring Arrays

Follow, or Monitor, mode provides some of mdadm's best and most unique features. Using Follow/Monitor mode you can daemonize mdadm and configure it to send email alerts to system administrators when arrays encounter errors or fail. You can also use Follow mode to arbitrarily execute commands when a disk fails. For example, you might want to try removing and reinserting a failed disk in an attempt to correct a non-fatal failure without user intervention.

The following command will monitor /dev/md0 (polling every 300 seconds) for critical events. When a fatal error occurs, mdadm will send an email to sysadmin. You can tailor the polling interval and email address to meet your needs.

# mdadm --monitor --mail=sysadmin --delay=300 /dev/md0

When using monitor mode, mdadm will not exit, so you might want to wrap it around nohup and ampersand:

# nohup mdadm --monitor --mail=sysadmin --delay=300 /dev/md0 &

Follow/Monitor mode also allows arrays to share spare disks, a feature that has been lacking in Linux
software RAID since its inception. That means you only need to provide one spare disk for a group of
arrays or for all arrays. It also means that system administrators don't have to manually intervene to shuffle around spare disks when arrays fail. Previously this functionality was available only using hardware RAID. When Follow/Monitor mode is invoked, it polls arrays at regular intervals. When a disk failure is detected on an array without a spare disk, mdadm will remove an available spare disk from another array and insert it into the array with the failed disk. To facilitate this process, each ARRAY line in /etc/mdadm.conf needs to have a spare-group defined.

In this example, both /dev/md0 and /dev/md1 are part of the spare group database. Just assume that /dev/md0 is a two-disk RAID-1 with a single spare disk. If mdadm is running in monitor mode (as I showed earlier), and a disk in /dev/md1 fails, mdadm will remove the spare disk from /dev/md0 and insert it into /dev/md1.

mdadm has many other options that I haven't covered here. I strongly recommend reading its manual page
for further details. Remember, you don't have to switch to mdadm. raidtools is still in development, and it
has the benefit of many years of development. But, I find that mdadm is a worthy replacement. It is both
feature rich and intuitive, and there's no harm in trying out alternatives.